Immediate-or-Cancel Order
An Immediate-or-Cancel (IOC) order is an order type that requires immediate full or partial execution and cancels the unfilled portion instantly, without allowing any part of the order to remain in the queue.
The Immediate-or-Cancel order type was among the most precisely defined order conditions available on Indian stock exchanges. When an IOC order was placed, the system attempted to match it against the best available counter orders in the order book immediately. Any quantity that found an immediate match was executed; the remaining unmatched quantity was cancelled instantaneously. Unlike a limit order that could sit in the book for the duration of the session, an IOC order had a guaranteed short life — it either traded immediately or was gone.
IOC orders were widely used by institutional investors and algorithmic trading systems that needed to execute large orders quickly without the risk of the order sitting exposed in the book and revealing trading intentions. A large fund manager reducing a position in a mid-cap stock might use an IOC order to capture whatever liquidity was available at a given price in a single sweep, without risking partial execution that lingered in the order book and signalled distress selling to other participants.
On NSE's NEAT and BSE's BOLT Plus trading systems, IOC orders were natively supported as a valid order condition since the early years of electronic trading in India. Retail investors also had access to IOC orders through standard broker terminals, although the use case for retail was different — an individual trader wanting to enter a position at a specific price during a fast-moving market might use an IOC to ensure they did not get filled at a worse price if the market moved away before their order could execute.
An important characteristic of IOC orders was that they required a specified price (limit IOC) or could be submitted as market IOC orders. A market IOC order swept through the order book consuming all available supply or demand at successive price levels until the quantity was filled or the book was exhausted, after which the remainder was cancelled. This could result in execution at multiple price points, with the average reported as the trade price.
Regulatory compliance was a consideration for IOC orders in the context of algorithmic trading. SEBI's algorithmic trading regulations required brokers to ensure that order rates and volume from algorithmic systems were within prescribed limits. High-frequency algorithms that placed and cancelled IOC orders in rapid succession were subject to SEBI's order-to-trade ratio norms, which required that a minimum proportion of submitted orders actually resulted in trades.